In a wireless communication system implementing link adaptation, a receiver such as a mobile terminal feeds back channel information to a transmitter such as a base station so that the transmitter can adapt its transmission to the receiver in dependence on channel conditions.
MIMO refers to the use of multiple transmit antennas and multiple receive antennas for the transmission of a signal in order to improve performance in a wireless communication system. A highly schematised block diagram of a MIMO system is shown in FIG. 1. The system comprises a transmitter 2 having multiple antennas 6(1) . . . 6(n) and a receiver 4 having multiple antennas 8(1) . . . 8(m). For example, in a cellular communication system like the 3GPP Long Term Evolution (LTE) standard, the transmitter 2 may be a base station (e.g. eNode-B in the 3GPP terminology) and the receiver 4 may be a mobile terminal (user equipment or UE in the 3GPP terminology). The transmitter 2 transmits a signal on some or all of its antennas 6, and the receiver 4 receives the signal on some or all of its antennas 8. To achieve good closed-loop performance, the transmitter 2 may perform MIMO “pre-coding” whereby it uses channel information to determine the relative amplitude and phase with which to transmit the signal on each antenna.
In general, this information has to be fed back from the receiver 4. To reduce the amount of feedback overhead, a precoding matrix approach was proposed in D. Love and R. W. Heath, “Limited Feedback Precoding for Spatial Multiplexing Systems”, in Proc. IEEE Globecom 2003, pp. 1857-1861. The basic idea behind this approach is to quantize the MIMO channel using a codebook consisting of a set of pre-defined matrices. For each channel realization, the receiver 4 finds the best precoding matrix (according to some performance criteria) from the codebook shared between the receiver and the transmitter, and then feeds back only the index of this matrix to the transmitter. This index may be referred to as a precoding matrix indicator (PMI).
Another piece of information that the receiver 4 feeds back to the transmitter 2 is the rank indicator (RI). This provides the rank of the channel matrix, which is defined as the number of linearly independent columns of the channel matrix. For example, a NT=4×NR=4 channel matrix can have rank equal to 4, 3, 2 or 1 (rank≦min (NT,NR)). The rank of the channel also determines the size of the precoding matrix to be used by the transmitter, i.e., the number of columns of the precoding matrix. Depending on the channel rank, the transmitter 2 will consider a specific subset of the full precoding codebook. Therefore, the transmitter 2 needs to know what rank the received PMI is referring to.
Further, in addition to the RI and PMI, the receiver 4 feeds back a channel quality indicator (CQI) to the transmitter 2, indicative of some metric relating to the received quality on the downlink channel. The transmitter 2 can then also take this into account when adjusting its transmission to the receiver 2, typically selecting the appropriate modulation scheme and code rate to match the receiver channel quality information.
As illustrated schematically in FIG. 2, the downlink channel may be an Orthogonal Frequency Division Multiplexing (OFDM) channel comprising a plurality of frequency sub-bands 12, with the sub-bands being grouped together into groups of sub-bands 14. The feedback of the CQI information may be either frequency selective or non frequency selective. In the non frequency selective case, the receiver 4 simply feeds back a single wideband CQI for the whole channel. In the frequency selective case, the receiver 4 also feeds back a CQI for each of a plurality of groups of sub-bands 14.
In the current 3GPP LTE standard, the rank indicator (RI), precoding matrix indicator (PMI) and channel quality indicator (CQI) are typically reported periodically from the UE to the eNode-B. This periodic reporting is based on a control signalling in the form of a set of parameters transmitted by the network via the eNode-B to the UE, which determine the periodicity of the different reports for a given feedback mode [3GPP TS 36.213, “Technical Specification Group Radio Access Network: Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8)”, V8.3.0, May 2008, Section 7.2.2].
For the non-frequency selective periodic CQI modes, the UE reports in different uplink reporting instances a) RI and b) wideband CQI/PMI for the modes with PMI report or only wideband CQI for the modes with no PMI report [3GPP TS 36.213, “Technical Specification Group Radio Access Network: Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8)”, V8.3.0, May 2008, Table 7.2.2-3].
For the frequency selective periodic CQI modes, the UE reports in different uplink reporting instances a) RI, b) wideband CQI/PMI for the modes with PMI report or only wideband CQI for the modes with no PMI report, and c) frequency selective CQI in terms of multiple sub-band CQIs [3GPP TS 36.213, “Technical Specification Group Radio Access Network: Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8)”, V8.3.0, May 2008, Table 7.2.2-3].
The control signalling from the eNode-B to the UE may be transmitted on the Primary Downlink Control Channel (PDCCH) and the RI, PMI and CQI reports fed back from the UE to the eNode-B may be signalled on the Primary Uplink Control Channel (PUCCH). An example of the RI, PMI and CQI information sent on the PUCCH 20 is illustrated schematically in FIG. 3a. Here, the PUCCH 20 comprises the sequential transmission in time on a plurality of reporting instances 22(t), 22(t+1), 22(t+2), etc. Here, the first reporting instance comprises a report of the RI, the second reporting instance comprises a report of the wideband PMI and wideband CQI, and by way of example the next four reporting instances comprise respective reports of the sub-band CQI values for each of four groups of sub-bands 14. Following the sub-band CQI reports, the sequence of an uplink reporting instance including the wideband PMI and CQI report followed by four reporting instances including sub-band CQI reports is repeated. That sequence may be repeated a number of times periodically, and after that the whole sequence may be repeated again periodically starting with another RI report and so on. The actual RI, PMI and CQI values reported will be updated with each periodic repetition on the relevant reporting instances. Note that FIG. 3a shows an example of a frequency selective report, but it will be understood that a non-frequency selective report would contain the same sequence of RI, PMI and CQI reports, except that it would not include the sub-band CQI values.
However, in some cases the UE may for certain reasons not transmit on one or more reporting instances 22 of the PUCCH 20. If an RI, PMI and/or CQI report is scheduled for such a reporting instance 22, then this RI, PMI and/or CQI report is said to be “dropped” and it will not be transmitted. There are also certain cases where a higher priority uplink transmission may cause an RI, PMI and/or CQI report to be replaced on a certain reporting instance 22. More specifically, when the UE has any other higher priority control information to be transmitted on the PUCCH, it will need to replace any RI, PMI and/or CQI report scheduled on that reporting instance 22. In such cases, the RI, PMI and/or CQI report is again said to be “dropped” from the reporting instance 22 in question.
The 3GPP LTE standard allows the possibility of dropping the transmission of RI and wideband CQI or wideband CQI/PMI from a given reporting instance 22 for different reasons:                An aperiodic CQI report on PUSCH is requested, which will be transmitted instead of the scheduled periodic CQI report on PUCCH.        A scheduling request (SR) needs to be transmitted by the UE, which will cause a drop of information on PUCCH.        A positive or negative acknowledgment (ACK/NACK) needs to be transmitted by the UE, which will cause a drop of information on PUCCH.        A UE Discontinuous Reception (DRX) inactive cycle will cause any uplink transmissions to be invalid (typically for power saving reasons).        RI and wideband CQUPMI collisions due to the RI offset parameter set to O=0 by the eNode-B, in which case the UE will drop the wideband CQI/PMI transmission.        In the presence of a measurement gap, the UE will drop all uplink transmissions overlapping with the gap.        
The missed transmission of this information in the uplink can cause a problem, because without the RI and/or PMI transmission, the CQI values sent on the following reporting instances have no meaning. In fact, all the RI/PMI/CQI reports are linked, and the wideband PMI is computed based on the reported rank while the sub-band CQI values are determined by the UE based on both the reported rank and precoding matrix. So the meaning of the reported PMI depends on the RI, and the meaning of the reported CQI depends on the RI and PMI. This implies that the eNode-B needs to know the correct RI in order to correctly interpret the reported PMI, and needs to know the correct RI and PMI in order to correctly interpret the reported CQI
The current status of the LTE specification is to do nothing and accept losing the RI or PMI information in the presence of a drop of a scheduled RI or PMI transmission.
A possible alternative solution is to configure the UE to reschedule the RI report by shifting it along in time to another reporting instance after the reporting instance at which it was originally scheduled. All subsequent reports are then also shifted along in time by the same number of reporting instances 22. This means that under normal circumstances, the eNode-B should still receive the RI correctly in order to interpret the subsequent PMI and CQI reports.
An example of this is illustrated in FIG. 3b, which shows the case of a DRX inactive cycle in which any uplink transmissions are invalid, or a measurement gap in which the uplink signal is not transmitted. Consider a scenario where a measurement gap or a UE DRX inactive cycle overlaps with a PUCCH reporting instance 22 containing an RI transmission, as depicted in the FIG. 3b (the DRX/GAP period can last multiple WB/CQI reporting intervals, but for illustration only one WB/CQI interval is shown as overlapping the DRX/GAP period). Under the current status of the LTE specification, any reports in the DRX/GAP period would simply be dropped and not retransmitted. But, under the possible alternative solution, the RI report is re-scheduled to the next available reporting instance 22(t+4) immediately after the end of the DRX/GAP period, and the subsequent sequence of PMI and CQI reports is shifted along in time accordingly.
Another example is illustrated in FIG. 3c, which shows the case where the UE receives data transmission from the eNode-B and in response must send back a positive acknowledgement signal ACK ora negative acknowledgement signal NACK to the eNode-B in the next reporting instance 22 of the PUCCH 20. That means that the RI, PMI or CQI report that was scheduled for that reporting instance must be dropped, since the ACK has higher priority than the RI, PMI and CQI reports. Again, under the current status of the LTE specification, that report would simply be omitted altogether and not retransmitted. This would include the possibility an RI report being replaced by the ACK/NACK. But, under the possible alternative solution, the RI report would be re-scheduled to the next reporting instance 22(t+1) immediately after the ACK/NACK, with the subsequent sequence of PMI and CQI reports being shifted along in time accordingly. Similar comments apply to any higher priority transmission that the UE must make to the eNode-B, which will displace an RI report.
Another alternative for the case of frequency-selective CQI report is to sacrifice one of the sub-band CQI reports every time a drop of RI or PMI transmission has occurred. Examples of this are illustrated in FIGS. 3d and 3e. In FIG. 3e for example the next sub-band CQI report CQI1 is deliberately omitted from transmission by the UE, and the eNode-B is configured to expect that CQI report CQI1 to be dropped. Similarly in FIG. 3d, the sub-band CQI report CQI3 is deliberately omitted from reporting instance 22(t+4), and the eNode-B is configured to expect that accordingly.
Yet another alternative would be to retransmit the dropped RI at the next opportunity, and to shift the subsequent sequence of PMI and CQI reports by one place, until the next wideband CQI/PMI reporting instance, thereby again sacrificing one of the sub-band CQI reports.
When an RI report is dropped, the current state of the LTE specification causes a problem because the eNode-B will lose the information of an entire reporting interval between one RI and the next.
However, the alternative solution discussed in relation to FIGS. 3b and 3c is also problematic because it can lead to a misalignment between eNode-B and UE in the interpretation of the different reports. For example, if a control signalling from the eNode-B is not properly detected by the UE, perhaps due to a poor quality PDCCH, then the UE may miss the transmission of downlink data, and not report the corresponding ACK/NACK in the uplink. In this case, there may be a discrepancy between what the UE transmits and what the eNode-B expects to receive. So referring to FIGS. 3b and 3c for example, the UE may transmit with the scheduling shown in the top row whilst the eNode-B expects to receive the scheduling shown in the bottom row. Thus the eNode-B's expectation will not be aligned with the UE's actual PUCCH transmission.
The alternative solution of FIGS. 3d and 3e reduces the impact of this misalignment problem to some extent. In FIG. 3e for example, the misalignment will always be regained again by reporting instance 22(t+3), and in FIG. 3d it will be regained by reporting instance 22(t+5). However, the situation in FIGS. 3d and 3e is still problematic in another way because it requires one of the sub-band CQI reports to be sacrificed.
It is an aim of the present invention to find an alternative solution to the problem of RI dropping.